Hydrogen has the highest energy density by mass of all common fuels but has one of the lowest energy densities by volume. Hydrogen storage technology, or the ability to safely carry enough hydrogen on-board a vehicle to enable at least a 300-mile range, is critical to the success of hydrogen fuel cell technology. At the present time, there does not exist a hydrogen storage solution in the art that meets the challenging performance requirements to make hydrogen-powered automobiles competitive with conventional vehicles.
In the past, several methods have been explored for the effective storage of hydrogen. Methods known in the art include pressurized storage of gaseous hydrogen, cryogenic storage of liquid hydrogen, storage of hydrogen by intercalation in chemical and metal hydrides, and the physisorption of hydrogen onto materials.
The mechanisms of sorption, the presence of atomic or ionic hydrogen in a crystal lattice or onto amorphous materials, as well as how defects and nano-scale effects can improve storage capacity are being actively pursued in the art of hydrogen storage technology.
Low cost and efficient storage of hydrogen are the primary requirements for successful hydrogen fuel cell technology. Temperature and pressure extremes, low volumetric density, and high diffusion rates resulting from heat leaks present obstacles in storing hydrogen cryogenically at high pressures. Fabricating tanks that are non-reactive and that can safely sustain extreme pressures and temperatures has proven to be a difficult challenge.
Other methods such as physisorption and intercalation of hydrogen are very cost effective and offer much smaller storage vessels in comparison with bulk hydrogen storage solutions.
Physisorption of hydrogen onto materials with large surface areas, mainly activated carbon and carbon nano-tubes and nano-structures, has been met with significant challenges such as low volumetric and gravimetric density and very low sorption temperatures.
Metal hydrides are chemical compounds formed when hydrogen gas reacts with metals. Metal hydrides store hydrogen interstitially, incorporating the hydrogen molecules into their crystal lattice. Intercalation of hydrogen in metal hydrides has proven to be a promising method for hydrogen storage. Hydrogen intercalation allows for large amounts of hydrogen to be stored at atmospheric temperatures and pressures. Previous advances in metal hydride storage have been limited to the larger, heavier elements of the lanthanide series and to the transition elements. Lightweight metal hydrides have become the current focus of interest and are viable options requiring further study to reach full potential. The most useful metal hydrides for hydrogen storage react near room temperature at hydrogen pressures a few times greater than the Earth's atmosphere.
Increasing the surface area of metal increases the absorption kinetics associated with hydrogen. A method known in the art for improving the surface area of metal is fabricating nano-crystalline powders that exhibit very high diffusion rates, high chemical activity, and high strength. To increase the surface area per gram of metal, different institutions have tested many unique approaches. Some of these approaches include, the use of nano-particles, sponges, micro-holes on a bulk metal, nano-films on substrates. Nano-films have significant advantages over the nano-particle solutions. Nano-films allow for a very large surface area to be achieved in a compact space. However, nano-films known in the art require the use of a supporting substrate. The substrate inherently adds weight to the film which is a disadvantage in most hydrogen storage applications.
Sputtering or physical vapor deposition (PVD) is a method of depositing that involves the removal of material from a solid cathode by bombarding it with positive ions from the discharge of a rare gas such as Argon (Ar). Sputtering is known in the art for the production of nano-films on substrates. The cathode may be made of a metal or an insulator, and, in contrast to thermal evaporation, complex compounds such as high temperature superconductor (HTS) materials can be sputtered with a lower degree of chemical compositional change. Sputtering is often done in the presence of a reactive gas, such as oxygen or nitrogen, to control or modify the properties of the deposited film. There are many advantages to metal deposition by sputtering techniques. These advantages include the ability to choose from of a wide range of deposition rates for best growth conditions, the ability to control a wide range of oxygen or nitrogen levels in the resultant dielectric films, the use of oxide or non-oxide targets (such as reactive sputtering deposition), the ability to grow C-axis oriented layers on amorphous substrates, and the ability to grow not only C-axis oriented, but also A-axis oriented layers on a single-crystalline substrate
Sputtering deposition systems provides high-density nucleation, which has not only C-axis but also A-axis orientation on single-crystalline substrates. This process is ideal for the first nucleation step. However the sputtering deposition method fails to make single crystal formation because of the difficulty in maintaining the correct stoichiometry at higher temperatures necessary to grow a single crystal. This is discussed in Onishi et al, “Chemical Vapor Deposition of Single-Crystalline ZnO Film with Smooth Surface on Intermediately Sputtered ZnO Thin Film on Sapphire”; Japanese Journal of Applied Physics, 1978, Vol 17, pp. 773-778, incorporated herein by reference. In order to deposit compound materials, alloy or mosaic targets are necessary. Two line sputtering sources can also be used for two-composition depositions. Multi-combination sputtering guns can be used to cancel or to trap electrons from the plasma so that the surface of the belt can be maintained at less than 90° C. This feature is described in Onishi et al, “Transparent and Highly Oriented ZnO Films Grown at Low Temperature by Sputtering with a Modified Sputter Gun”; Applied Physics Letters, 1981, Vol. 38, pp. 419-421, incorporated herein by reference.
Accordingly, what is needed in the art is a hydrogen storage nano-foil hydride that exhibits improved sorption and desorption qualities at desirable temperatures which is also light weight and easily adaptable for use as a hydrogen storage solution.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.